"""Entity restore registry — declarative configuration for undo/restore.

Each registered entity type declares:
- model_class: SQLAlchemy model to load
- restore_permission: permission string required to restore
- excluded_fields: fields never restored from snapshot (id, tenant_id, timestamps, etc.)
- special_handler: optional async callable for entity-specific restore logic

No dynamic ORM loading, no blind snapshot writes — only explicitly registered
entity types can be restored, and only through their declared configuration.
"""

from __future__ import annotations

import logging
from collections.abc import Awaitable, Callable
from dataclasses import dataclass, field
from typing import Any

from sqlalchemy.ext.asyncio import AsyncSession

logger = logging.getLogger(__name__)

# Default fields excluded from restore for every entity type
_DEFAULT_EXCLUDED = frozenset({
    "id",
    "tenant_id",
    "created_at",
    "updated_at",
    "deleted_at",
    "search_tsv",
    "embedding",
})

# Type alias for special restore handler
SpecialRestoreHandler = Callable[
    [AsyncSession, Any, str, Any, dict[str, Any]],
    Awaitable[dict[str, Any]],
]


@dataclass
class RestoreConfig:
    """Configuration for restoring a specific entity type."""

    entity_type: str
    model_class: type
    restore_permission: str
    excluded_fields: frozenset[str] = field(default_factory=frozenset)
    special_handler: SpecialRestoreHandler | None = None

    @property
    def all_excluded_fields(self) -> frozenset[str]:
        """Merge default excluded fields with entity-specific ones."""
        return _DEFAULT_EXCLUDED | self.excluded_fields


class RestoreRegistry:
    """Singleton registry mapping entity_type → RestoreConfig."""

    _instance: RestoreRegistry | None = None

    def __new__(cls) -> RestoreRegistry:
        if cls._instance is None:
            cls._instance = super().__new__(cls)
            cls._instance._configs: dict[str, RestoreConfig] = {}
        return cls._instance

    def register(self, config: RestoreConfig) -> None:
        """Register a RestoreConfig for an entity type."""
        if config.entity_type in self._configs:
            logger.warning("Overwriting restore config for entity_type: %s", config.entity_type)
        self._configs[config.entity_type] = config
        logger.debug("Registered restore config for: %s", config.entity_type)

    def unregister(self, entity_type: str) -> None:
        """Remove a RestoreConfig (e.g. when plugin is deactivated)."""
        if entity_type in self._configs:
            del self._configs[entity_type]
            logger.debug("Unregistered restore config for: %s", entity_type)

    def get(self, entity_type: str) -> RestoreConfig | None:
        """Get RestoreConfig for entity_type, or None if not registered."""
        return self._configs.get(entity_type)

    def is_registered(self, entity_type: str) -> bool:
        """Check if entity_type is registered for restore."""
        return entity_type in self._configs

    def list_registered(self) -> list[str]:
        """Return all registered entity types."""
        return sorted(self._configs.keys())

    def _reset_for_testing(self) -> None:
        """Clear all registrations — for unit tests only."""
        self._configs.clear()


def get_restore_registry() -> RestoreRegistry:
    """Return the global RestoreRegistry singleton."""
    return RestoreRegistry()


def reset_restore_registry_for_testing() -> RestoreRegistry:
    """Return a fresh singleton — for unit tests only."""
    reg = get_restore_registry()
    reg._reset_for_testing()
    return reg


# ─── Default entity registrations ───


def register_default_entities() -> None:
    """Register Core entity types for restore.

    Called during app startup. Plugin entities register themselves
    in their on_activate() lifecycle hook.
    """
    # Contact is registered by ContactsPlugin.on_activate() — do not register
    # it here to avoid double registration. This function remains for future
    # Core entities that have no plugin.

    # Plugin entities (task, calendar_entry, dms_file, mail) are registered
    # by their respective plugins in on_activate(). See P0-7 fix.